The present invention relates to a fuel cell apparatus, and in particular, to an improvement of a so-called PEM type fuel cell apparatus having a polymer solid electrolyte film. More particularly, the present invention relates to an improvement of a water direct injection type, in particular, to a fuel cell apparatus with direct spraying of water onto an air electrode from a nozzle.
A cell main body of the PEM type fuel cell apparatus has a structure including a polymer solid electrolyte film held between a fuel electrode (also called as a hydrogen electrode in the case of using hydrogen as a fuel electrode) and an air electrode (also called an xe2x80x9coxygen electrodexe2x80x9d or xe2x80x9coxidation electrodexe2x80x9d because oxygen is a reaction gas. A reaction layer including a catalyst is interposed between the air electrode and the electrolyte film.
The fuel cell having the above structure is generated in an electromotive force in the following manner. More specifically, a fuel gas is supplied to a fuel electrode side (anode), and then, an oxidation gas is supplied to an air electrode side; as a result, electricity is generated with the progress of electrochemical reaction, and then, the electrocity thus generated is picked up by an external circuit.
More specifically, a hydrogen ion obtained by the fuel electrode (anode) is moved in the form of a ion (H3O+) to the air electrode (cathode) side in an electrolyte film containing water. Moreover, an electron obtained by the fuel electrode (anode) is moved to the air electrode (cathode) side through an external load, and then reacts with oxygen contained in an oxidation gas (e.g., air) to generate water. Thus, electric energy generated by consecutive electrochemical reactions.
The present applicant previously proposed a fuel cell apparatus in Japanese Application No. 10-378161. The fuel cell apparatus has a structure supplying liquid water onto the surface of air electrode for the purpose of cooling the air electrode having an exothermic reaction so as to improve power generation performance.
In a so-called water direct injection type fuel cell apparatus as proposed in the above application, feed water is controlled in accordance with temperature of the fuel cell main body so as to cool the fuel cell main body. On the other hand, a predetermined amount of process air is constantly supplied to the air electrode. In other words, the air volume delivered by the air supply system is always constant. Applicants"" prior application describe influence of the sensible heat and latent heat of the water cooling the fuel cell main body. In this case, the sensible heat is that heat which is removed from the fuel cell main body without vaporization of the supplied water. On the other hand, the latent heat is heat which is removed from the fuel cell main body by vaporization of the directly injected water.
It has now been found that the latent heat of water is used to cool the fuel cell main body, and that the sensible heat makes little contribution to cooling. Therefore, in order to more effectively use the latent heat of water, in other words, in order to more effectively cool by vaporizing water supplied to the air electrode, supply amount of process air supplied to the air electrode, that is, the air volumetric flow rate should be controlled. Given, such insight applicants now recognize a number of deficiencies in the previously proposed water direct injection type fuel cell apparatus.
More specifically, when the fuel cell main body is operated at a high temperature, unless the amount of air (predetermined amount of supply) supplied to the air electrode is sufficient to properly utilize the latent heat of water, the fuel cell dries up and for this reason, the air temperature becomes high. In such a case, in order to cool the fuel cell main body, a large amount of water is supplied so as to utilize the latent heat of vaporization of water. However, in this case, a large capacity pump is required for supplying the large amount of water. The large capacity pump hinders any attempt to miniaturize the fuel cell apparatus, and a great amount of power is consumed in driving the large capacity pump, thus reducing the efficiency of the fuel cell apparatus. Moreover, when a large amount of water is supplied to the fuel cell, its process air passage fills with water, or a water membrane is formed on the surface of the air electrode, creating the possibility that the amount of oxygen necessary for the chemical reaction of the fuel cell will not be supplied to the air electrode.
On the other hand, when the fuel cell main body is operated at a low temperature, in the case where the air (predetermined amount) supplied to the air electrode is excessive, the temperature of the fuel cell main body is lowered, and there is a power loss for the fan which supplies the air.
The water evaporated at the air electrode is condensed for recycle by a condenser together with reaction water and, thereafter, is recovered. The condenser can effectively condense water when only a small amount of air is to be treated and the temperature of the air is high, in which case the capacity of the condenser can be small and the condenser small is size. In the case where the fuel cell main body is operated at a low temperature and the supply of process air is larger, a larger capacity (large size) condenser is required.
The present invention has been made taking the above-described problem in the prior art into consideration. It is, therefore, an object of the present invention to provide a fuel cell apparatus, which includes a water supply for supplying water, in liquid form, onto a surface of an air electrode of a fuel cell.
The fuel cell apparatus of the present invention further includes an air supply controller for varying the amount of process air supplied to the air electrode.
In the fuel cell apparatus constructed as described above, the amount of process air (volumetric flow rate) is variable so that it can be set to the optimum amount, whereby it is possible to sufficiently and effectively cool using the latent heat of evaporation of water supplied to the air electrode, i.e., to effectively cool the air electrode, in particular, and the fuel cell body, in general. The droplet size of the water spray ranges from 50 xcexcm to 500 xcexcm in order to most effectively use latent heat of evaporation of the water. Moreover, it is desirable that the thickness of the electrolyte film of the fuel cell be less than 200 xcexcm.
More specifically, when the fuel cell main body is operated at a high temperature to reduce the temperature, the amount of air supplied per unit time, (the amount of air passing through the air chamber Axe2x80x94see FIG. 3) is increased, taking caution that a sufficient amount of water is supplied. In prior art apparatus wherein the supply of air is fixed the sensible heat of water is used, and a relatively large quantity of water must be supplied and for this reason, there are various problems even if the amount of air supplied is increased. In the present invention, however, almost no problem is caused even if the amount of air supplied is great. Even then, the load on the air supply device (fan, etc.) is extremely small as compared with the prior art which uses a greater amount of water.
When the fuel cell is operated at a low temperature to increase its operating temperature, the amount of the air supply is decreased. By doing so, it is possible to securely increase the temperature of the fuel cell main body, while reducing the power consumed by the air supply device to the extent possible.
Moreover, in the water recycle condenser, as the internal air temperature increases, the temperature difference between the internal and external air increases and, therefore, the capacity of the condenser can be made smaller.
According to the present invention, the air supply and the water supply are controlled independently of each other. Therefore, it is possible to independently control the required amounts of the air and water with the required timing. By doing so, it is possible to effectively obtain a high output from the fuel cell without being wasteful. Further, the amounts of air and water exiting the fuel cell apparatus are minimized and, therefore, it is possible to make the condenser small in size, and to reduce the power consumption by auxiliary equipment. Furthermore, it is possible to shorten the time required for start-up.
FIG. 1 is a graph showing the relationship between a load (current density) of the fuel cell apparatus and exhaust air temperature for various stoichiometric ratios. The stoichiometric ratio is a predetermined amount of air supplied to the air electrode using the amount of process air including oxygen theoretically consumed in the fuel cell reaction as a reference. Therefore, in the case of the stoichiometric ratio 1, the theortectical required minimum amount of air is supplied. In the case of the stoichiometric ratio 2, the amount of air supplied is twice that of stoichiometric ratio 1.
As seen from the graph shown in FIG. 1, as the stoichiometric ratio becomes smaller, that is, as the amount of air supply is reduced, the fuel cell apparatus is operated at a higher temperature in order to obtain the same load. The higher the operating temperature of the fuel cell apparatus, the higher the efficiency becomes. Moreover, the exhaust air temperature is increased by the high temperature operation, so that the capacity of the condenser can be made smaller. Therefore, it is preferable that the fuel cell main body be operated at the highest temperature maintaining a required load. The load and the temperature of the fuel cell main body are uniquely determined by the stoichiometric ratio; therefore, one of the load and temperature is monitored, and then, the stoichiometric ratio, that is, the amount of air supply, more specifically, the air flow rate at the air chamber inlet, is determined.
However, in a conventional fuel cell, there are various limits on the operating temperature of the fuel cell main body and on the stoichiometric ratio (amount of air supply). For example, in order to reliably prevent the fuel cell main body from becoming burned, the operating temperature of the fuel cell main body needs to be set to 100 to 80xc2x0 C. or less, for example. Moreover, according to the research by the present inventors, operation of the fuel cell main body was impossible under the conditions on the upper side of the broken line L shown in FIG. 1. It is theorized that this observed inoperativeness is due to the following reasons. More specifically, when the amount of air supply is small (when the volume of air is small), air is not efficiently supplied to the air electrode due to resistance within the air supply passage and the gas diffusion layer, catalyst powder and the like.
Therefore, in FIG. 1, for example, the fuel cell main body is operable in a range of 80xc2x0 C. or less and on the lower side of the broken line L. Considering its efficiency, it is preferable that the fuel cell main body be operated at the highest temperature in the above operable range.
In a vehicle fuel cell apparatus having a severe load fluctuation, the amount of air supply is changed in accordance with the required load. At that time, simultaneously, the temperature of the fuel cell main body is detected, and then, preferably, the amount of air supply is adjusted so that the highest temperature realizing the required load, that is, the minimum stoichiometric ratio can be obtained.
On the other hand, if the fuel cell apparatus is used in an environment wherein there is no load variation, only temperature of the fuel cell main body need be monitored, and then, only when the temperature changes is the amount of air supply adjusted so that the temperature is controlled as desired. More specifically, where the temperature of the fuel cell main body becomes lower than a desired temperature range, the amount of air supply is decreased so as to reduce the cooling effect of the latent heat of evaporation of water. On the other hand, in the case where the temperature of the fuel cell main body becomes higher than a desired temperature range, the amount of air supply is increased so as to enhance the cooling effect of the latent heat of evaporation of water.
The external environment and the performance of auxiliary equipment, impose various limits on the operating conditions of the fuel cell apparatus. The operating conditions of the fuel cell main body are limited to a range indicated by the square in the operable conditions shown in FIG. 1. In this range, the operating temperature of the fuel cell main body does not exceed the line of the stoichiometric ratio 1. The amount of air supply is always maintained at least at the amount corresponding to stoichiometric ratio 1 in order to ensure continuous operation of the fuel cell. Therefore, there is no need for monitoring the temperature of the fuel cell. Accordingly, only load is monitored so that the minimum amount of air capable of outputting the required load is supplied.
In all of the scenarios described above, the amount of water is continuously supplied to the air electrode is sufficient to allow for the water which is vaporized by the heat of the fuel cell and to ensure that liquid water is always present on the air electrode and in its surroundings (i.e., air chamber) during an operation of the fuel cell apparatus.
As described above, water always present in the air electrode, therefore, the latent heat of vaporization of water can be effectively used. As a result, it is possible to reduce the cooling plates in a stack of the fuel cell main body, or to omit the cooling plate altogether. However, where it is impossible to reliably provide for vaporization of a sufficient amount of water, it is preferable that the stack of the fuel cell main body be provided with a cooling plate, cooling pipe or other cooling device. The heat generated within the stack is removed to the exterior by a heat medium (usually, water) circulating through the cooling device, and the heat thus removed may be used for interior heating or the like (co-generation).
In the operation described above, the process air is substantially uncompressed, as supplied to the air electrode. However, the present invention may be applied to a fuel cell apparatus which includes a pressurized oxidizing gas supply system. The pressurized oxidizing gas supply may include a compressor or the system may become a pressurized (higher than atmospheric pressure) simply by resistance to gas flow within the system piping.
The temperature of the fuel cell main body may be measured by a thermometer attached to the fuel cell main body. As shown in FIG. 1, the temperature of exhaust air is measured, and thereby, it is possible to indirectly measure the temperature of the fuel cell main body. It is preferable to measure the temperature of the air just after being exhausted from the fuel cell main body.
The load of the fuel cell main body is a product of current and the voltage between its electrodes. The actual load presently output by the fuel cell main body is detected, and then, the detected load is used as a reference parameter to control the amount of process air. The demanded load for the fuel cell apparatus is detected, for example, as a speed, torque or accelerator opening, and then, used as the control parameter.